<!DOCTYPE html>
<html>
<body>
    <h1>JavaScript 回调</h1>
    <p>等待 3 秒（3000 毫秒）让此页面发生变化。</p>
    <h1 id="demo"></h1>
    <script>
        setTimeout(myFunction, 3000);
        function myFunction() {
            document.getElementById("demo").innerHTML = "Hello!!";
        }
    </script>
</body>
</html>